Method and Apparatus for Selecting an Optimum Write Parameter, and Optical Recording Medium for Use by Said Method and Apparatus

ABSTRACT

The invention relates to a method for setting an optimum value of a write parameter for use in an optical recording apparatus for writing information on an optical recording medium by means of a radiation beam. The optimum value of a write parameter is found by curve-fitting a function and obtaining a characteristic write power level (P Char ) from the curve-fitting function. It is then assessed if the characteristic write power level (P Char ) qualify as an optimum value of a write parameter. If the characteristic write power level (P char ) does not qualify as an optimum value of a write parameter then an iteration procedure is started, where the subsequent initial values of write power level (P ini , n), are given by Pini, n+1=A P ini , n+(1−A)P char , n, where A is a constant, and n is an integer. The invention also relates to an optical recording apparatus and an optical recording medium according to the invention.

The invention relates to a method for setting an optimum value of awrite parameter for use in an optical recording apparatus for writinginformation on an optical recording medium by means of a radiation beam.The invention also relates to an optical recording apparatus forrecording information on an optical recording medium, the apparatuscomprises a radiation source for emitting a radiation beam having acontrollable value of a write power level for recording information onthe recording medium. Additionally, the invention relates to an opticalrecording medium for recording information by irradiating the recordingmedium by means of a radiation beam, the recording medium comprising anarea containing control information indicative of a recording processwhereby information can be recorded on said recording medium.

In an optical recording apparatus it is important to record informationon optical recording media with the correct power of the irradiationbeam, such as for example a laser beam. Though a media manufacturer cangive this correct power in an absolute way (for example, pre-recorded onthe disc) environment and apparatus-to-apparatus deviations for everyrecording medium and recording apparatus combination makes it necessaryfor reliable optical recording to find the correct power dependent ofthe recording medium in question. Several optimization power control(OPC) routines for this purpose are known in the field, such as thegamma-OPC (generally used for rewritable media such as for example CR-RWand DVD+/−RW), the beta-OPC (generally used for recordable media such asfor example CD-R and DVD+/−R), and the kappa-OPC (for example used forBD-RE/R media).

In WO 02/41306, which is hereby incorporated in the present applicationby reference and considered part of the present application, methods aredescribed in which an optimum write power of a radiation beam in anoptical recording apparatus is set by writing a series of test patternson the optical recording medium, forming a read signal from the patternsand processing the read signal. Such processing involves fitting afunction, preferably a straight line, to parameters obtained from theread signal without having to perform a differentiation step. One suchparameter obtainable from the read signal is the modulation (M). Thecurve fitting procedure requires one or more power values to define theinterval wherein the fitting is to be performed. If the curve fittingresults in an first value for a write parameter that does not qualify asthe optimum write parameter for some reasons, e.g. out of range of apre-set value, WO 02/41306 discloses several possible measures to betaken. One such being an iteration procedure, wherein said first valuefor a write parameter is used to define a second interval wherein thefitting is to be performed. The resulting second write parameter maysubsequently be evaluated as an optimum write parameter, e.g. comparedto a pre-set range. If the second write parameter does not qualify asthe optimum write parameter the iteration may continue on until somepre-set condition is meet.

However, an inherent problem for some combinations of optical recordingapparatus and optical recording media is that the iteration may need arelative large amount of iteration steps before an optimum writeparameter is obtained. Hence, the recording process may be delayedcausing some user discomfort. Moreover, in some unfortunate combinationsof optical recording apparatus and optical recording media the iterationmay not be convergent towards an optimum write parameter. For suchcases, the optical recording apparatus will not be able to record on therecording media resulting in a malfunction.

Hence, an improved optical recording apparatus would be advantageous,and in particular a more efficient and/or reliable optical recordingapparatus would be advantageous.

Accordingly, the invention preferably seeks to mitigate, alleviate oreliminate one or more of the above-mentioned disadvantages singly or inany combination. In particular, it may be seen as an object of thepresent invention to provide a method that solves the above mentionedproblems of the prior art when obtaining an optimum value of a writeparameter for optical recording on an optical media. From said optimumvalue of a write parameter, an optimum value (P_(opt)) of the writepower level (P) may be derived.

This object and several other objects are achieved in a first aspect ofthe invention by providing a method for setting an optimum value of awrite parameter for use in an optical recording apparatus for writinginformation on an optical recording medium by means of a radiation beam,said method comprising the steps of:

1) writing a series of test patterns on the recording medium, eachpattern being written with a different value of a write power level (P)of the radiation beam,2) reading the patterns so as to form corresponding read signal portions(18, 19),3) deriving a value of a read parameter from each read signal portion,4) curve-fitting a function defining a relation between the readparameter and the write power level (P), to associated values of theread parameter and the write power level (P), the curve-fitting functionhaving a initial value of write power level (P_(ini,)),5) obtaining a characteristic write power level (P_(char)) from thecurve-fitting function, and assessing if the characteristic write powerlevel (P_(char)) qualify as an optimum value of a write parameter, andif the characteristic write power level (P_(char)) does not qualify asan optimum value of a write parameter then start6) obtaining one or more characteristic write power levels (P_(char, n))by an iteration procedure where the subsequent initial values of writepower level (P_(ini, n)), are given by

P _(ini, n+1) =AP _(ini, n)+(1−A)P _(char, n),

where A is a constant, and n is an integer.

The invention is particularly but not exclusively advantageous forobtaining at least one optimum value of a write parameter withinrelatively short time. Beneficially, problems with non-convergent orunstable iterations are usually avoided. Thus, an improved and efficientmethod for obtaining a optimum value of a write parameter is provided.

The optimum value represents an optimum value obtained by the method ofthe present invention, but nevertheless the optimum value of the writeparameter found may not be the very best value of the write parameterobtainable, e.g. a local optimum and not a global optimum may beobtained.

The read parameter may be a modulation (M) of the amplitude of a readsignal derived from information recorded on the recording medium. Thismodulation (M) is computed from the following expression

M=((I _(H) −I _(L))/I _(H))·100,

where I_(H) is the highest level of the amplitude and I_(L) is thelowest level of the amplitude in the read signal derived from readinginformation recorded on the information carrier comprising longer markssuch as, for example, marks having a length of 14 times the channel bitlength when Eight-to-Fourteen Modulation Plus (EFM+) coding is employed.

A function for curve-fitting may be of the form

P·M==α·(P−β),

wherein α and β have values resulting from the curve-fitting. Thecharacteristic value of the write parameter (P_(char)) for the functionmay be set to be substantially equal to the value of β times a firstmultiplication constant (κ). Generally speaking, any function ofarbitrary shape that defines a relation between the read parameter andthe write power level (P) can be used. However, it should be noted thata straight line could be very easily and accurately curve-fitted.Therefore, arranging the values of the read parameter and of the writepower level (P) in such a way that a straight line could be curve-fittedis advantageous. Additionally, the value of β is unambiguouslydetermined from the straight line fitting procedure, which is quiteadvantageous relative to alternative fitting procedures where severalsolutions may result.

The first multiplication constant (κ) may be read from an area on therecording medium containing control information indicative of arecording process whereby information can be recorded on said recordingmedium. Thus, the media manufacturer may provide this information on theoptical medium. Alternatively, the multiplication constant (κ) may bedetermined by the user in a calibration procedure.

The curve-fitting of the function defining a straight line may becarried out in a predetermined fit range of write power levels, wherethe position of the predetermined fit range may be determined by theinitial value of the write power level (P_(ini)) associated with eachcurve-fitting function and a range factor (R), where R is a percentagenumber. The upper limit of the fit range may be given by P_(ini) times(1+R), and the lower limit of the fit range may be given by P_(ini)times (1−R). Values of the range factor (R) may be 10%, 20%, or 30%.

The information indicative of the initial write power level may be readfrom an area on the recording medium containing control informationindicative of a recording process whereby information can be recorded onsaid recording medium. Thus, the media manufacturer may provide thisinformation on the optical medium in order to facilitate an fast andaccurate determination of the optimum write power level (P_(opt)) forrecording information for the optical medium in question.

The constant A may be any value between 0 and 1, such as 0.1, 0.2, 0.3,0.4, 0.5, 0.6, 0.7, 0.8, or 0.9. The constant A may have a value in anyof the following intervals; 0.1-0.4, 0.2-0.3, 0.6-0.9, or 0.7-0.8.Alternatively, the constant A may have a value in any of the followingintervals; 0.15-0.35, 0.05-0.495, 0.01-0.499, 0.515-0.985, 0.505-0.995,or 0.501-0.999.

The method of the invention may provide a method for setting an optimumvalue (P_(opt)) of a write power level (P), of a radiation beam, whichmethod is intended for use in an optical recording apparatus for writinginformation on an optical recording medium by the radiation beam havingthe write power level (P), using a method as describe above for settingan optimum value of a write parameter, wherein the optimum value(P_(opt)) of the write power level (P) is set to be equal to the optimumvalue of the write parameter times a second multiplication constant (ρ).Notice, that this definition of the optimum value (P_(opt)) of the writepower level (P) is different from the disclosure of WO 02/41306, i.e.the multiplication constants are defined differently. The optimum value(P_(opt)) of the write power level may be defined as the write powerlevel for which the lowest jitter of the read signal from informationrecorded on the recording medium is obtained. Additionally, the optimumvalue (P_(opt)) of the write power level may take into account thestability of the read signal against variations around the optimum value(P_(opt)) of the write power level. The optimum value (P_(opt)) of thewrite power level may differ depending on the purpose of the recordingmedium. Thus, the optimum value (P_(opt)) of the write power level forone time recording may be different from the optimum value (P_(opt)) ofthe write power level for multiple recordings on the same region of therecording medium.

The second multiplication constant (ρ) may be read from an area on therecording medium containing control information indicative of arecording process whereby information can be recorded on said recordingmedium. Thus, the optical medium manufacturer may under standardizedcondition determine the second multiplication constant (ρ) and providethis information on the optical medium.

In a second aspect, the invention provides an optical recordingapparatus for implementing the method according to the first aspect ofthe invention, the apparatus comprising:

a radiation source for emitting a radiation beam having a controllablevalue of a write power level (P) for recording information on therecording medium,

a control unit for recording a series of test patterns, each patternbeing recorded with a different value of the write power level,

a read unit for reading the patterns and forming corresponding readsignal portions,

first means for deriving a value of a read parameter from each readsignal portion,

second means for curve-fitting a function defining a relation betweenthe read parameter and the write power level (P), to associated valuesof the read parameter and the write power level (P), the curve-fittingfunction having a initial value of write power level (P_(ini,)),

third means for obtaining a characteristic write power level (P_(char))from the curve-fitting function, and assessing if the characteristicwrite power level (P_(char)) qualify as an optimum value of a writeparameter, and

if the characteristic write power level (P_(char)) does not qualify asan optimum value of a write parameter then initiate,

fourth means for obtaining one or more characteristic write power levels(P_(char, n)) by an iteration procedure where the subsequent initialvalues of write power level (P_(ini, n)), are given by

P _(ini, n+1) =AP _(ini, n)+(1−A)P _(char, n)

where A is a constant, and n is an integer.

It is particularly beneficial that the present invention may be appliedby means already known in the art, though the way of finding the optimumvalue of a write parameter of the present invention has severaladvantages relative to the known way of finding the optimum value of awrite parameter.

In a third aspect the invention provides an optical recording medium forrecording information by irradiating the recording medium by means of aradiation beam, the recording medium comprising an area containingcontrol information indicative of a recording process wherebyinformation can be recorded on said recording medium, the controlinformation comprising values of recording parameters for the recordingprocess, wherein the control information comprises a informationindicative of the initial write power level (P_(ini)) for use in themethod according to the first aspect of the invention or the apparatusaccording to the second aspect of the invention.

It is particularly advantageous that the control information may beprovided on the recording medium itself as this enable the mediummanufacturer to distribute the control information to the user in anefficient manner.

Furthermore, the optical recording medium for recording information byirradiating the recording medium by means of a radiation beam maycomprise an area containing control information indicative of arecording process whereby information can be recorded on said recordingmedium, the control information comprising values of recordingparameters for the recording process, wherein the control informationcomprises at least one of the constant A, the first multiplicationconstant (κ) or the second multiplication constant (ρ) for use in themethod of the invention.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter. Thepresent invention will now be explained, by way of example only, withreference to the accompanying figures, where

FIG. 1 is a diagram of an embodiment of an optical recording apparatusaccording to the invention,

FIG. 2 illustrates two read signal portions from two test patterns,

FIG. 3 is a graph showing the measured modulation times the write poweras a function of the write power and two curve-fitted functions, and

FIG. 4 is a flow-chart of a method according to the invention.

FIG. 1 shows an optical recording apparatus and an optical recordingmedium 1 according to the invention. The recording medium 1 has atransparent substrate 2 and a recording layer 3 arranged on it. Therecording layer 3 comprises a material suitable for recordinginformation by means of a radiation beam 5. The recording material maybe of, for example, the magneto-optical type, the phase-change type, thedye type, metal alloys like Cu/Si or any other suitable material.Information may be recorded in the form of optically detectable regions,also called marks, on the recording layer 3. The apparatus comprises aradiation source 4, for example a semiconductor laser, for emitting aradiation beam 5. The radiation beam is directed to the recording layer3 via a beam splitter 6, an objective lens 7 and the substrate 2. Therecording medium may alternatively be air-incident, the radiation beamthen being directly incident on recording layer 3 without passingthrough a substrate. Radiation reflected from the medium 1 is collectedby the objective lens 7 and, after passing through the beam splitter 6,falls on a detection system 8 which converts the incident radiation inthe electric detector signals. The detector signals are applied to acircuit 9. The circuit 9 derives several signals from the detectorsignals, such as a read signal SR representing the information beingread from the recording medium 1. The radiation source 4, the beamsplitter 6, the objective lens 7, the detection system 8 and circuit 9together form a read unit 90.

The read signal from the circuit 9 is processed in a first processor 10in order to derive signals representing a read parameter from the readsignal. The derived signals are fed to a second processor 101 andsubsequently to a third processor 102 which processors process a seriesof values of the read parameter and derive therefrom a value for a writepower control signal necessary for controlling the laser power level.

The write power control signal is applied to a control unit 12. Aninformation signal 13, representing the information to be recorded onthe recording medium 1, is also fed to the control unit 12. The outputof the control unit 12 is connected to the radiation source 4. A mark onthe recording layer 3 can be recorded by a single radiation pulse, thepower of which is determined by the optimum write power level (P_(opt))as determined by the processor 102. Alternatively, a mark can also berecorded by a series of radiation pulses of equal or different lengthand one or more power levels determined by the write power signal.

A processor is understood to mean any means suitable for performingcalculations, for example a microprocessor, a digital signal processor,a hard-wired analog circuit or a field programmable circuit. Moreover,the first processor 10, the second processor 101 and third the processor102 may be separate devices or, alternatively, may be combined into asingle device executing all three processes.

Before recording information on the medium 1 the apparatus sets itswrite power (P) to the optimum value (P_(opt)) by performing a methodaccording to the invention. This method is schematically depicted in theflow-chart shown in FIG. 4.

In a first step 41 the apparatus writes a series of test patterns on themedium 1. The test patterns should be selected so as to give a desiredread signal. If the read parameter to be derived from the read signal isthe modulation (M) of a read signal portion pertaining to a testpattern, the test pattern should comprise marks sufficiently long toachieve a maximum modulation of the read signal portion. When theinformation is coded according to the so-called Eight-to-FourteenModulation (EFM), the test patterns preferably comprise the long 111marks of the modulation scheme. When the information is coded accordingto the Eight-to-Fourteen Plus Modulation (EFM+), the test patternsshould comprise the long 114 marks of this modulation scheme. When theinformation is coded according to the so-called 17PP modulation, thetest patterns preferably comprise the long 18 marks of the modulationscheme. Each test pattern is recorded with a different write power level(P). The range of powers can be selected on the basis of an indicativepower level (P_(ini)) recorded as control information on the recordingmedium. Subsequent test patterns may be recorded with a step-wiseincreased write power level (P) under the control of the control unit12. The test patterns may be written anywhere on the recording medium.They can alternatively be written in specially provided test areas onthe recording medium.

In a second step 42 the recorded test patterns are read by the read unit90 so as to form a read signal SR. FIG. 2 shows the read signal portions18 and 19 obtained from two test patterns written at two different writepower levels. The patterns shown comprise a short mark, a long mark anda short mark, as denoted by the signal parts 15, 16 and 17,respectively, in both the read signal portion 18 and the read signalportion 19. An actual pattern may comprise a few hundred marks ofdifferent or equal length.

In a third step 43 the processor 10 derives from the read signal SR aread parameter for each read signal portion. A possible read parameteris the ratio of the lowest level of the amplitude of a read signalportion (for read the signal portion 18 indicated by ‘a’ in FIG. 2) tothe maximum level of the amplitude of the same read signal portion(indicated by ‘b’). A preferred read parameter is the modulation (M),being the ratio of the maximum peak-to-peak value of a read signal,indicated by ‘c’, to the maximum amplitude ‘b’ of the read signalportion.

In a fourth step 44 the processor 101 fits a function to associatedvalues of the read parameter and the write power level (P). The functiondefines a relation between the read parameter and the write power level(P). As input for a curve fitting function a corresponding initial valueof write power level (P_(ini)) is provided. The write power can be takenfrom the value of the write power control signal during the recording ofthe test patterns or, alternatively, from a measurement of the radiationpower. The curve-fitting function may be of many different mathematicalforms readily available for a person skilled in the art. Thus, a curve,e.g. a polynomial, may be fitted to the power levels around the initialwrite power level (P_(ini)) in question.

In FIG. 3 a graph with a preferred embodiment of the curve-fittingprocedure is shown.

The vertical axis indicates the write power (P) times the modulation(M), whereas the horizontal axis indicates the write power (P). Each ofthe points 51 represents a pair of values for the measured modulation(M) times the write power (P) and the write power (P), respectively.

In the preferred embodiment of FIG. 3 an initial write power levelP_(ini,1) is shown. In a pre-defined interval around the initial writepower level P_(ini,1), indicated schematically by the arrow 52, themeasured points 51 of the graph in FIG. 3 is fitted to a function of theform

P·M=α·(P−β),

where α and β are fitting constants. The fitting function is a straightline 53 in the graph of FIG. 3. The values of β correspond to theintersection 54 of the line 53 with the horizontal axis. The straightline 53 and the associated constants α and β, are to be found by theleast-square method or other appropriate mathematical methods.

The interval 52 may be defined as a range fraction (R) of the initialswrite power levels (P_(ini)), e.g. 10%, 20%, 30% or 40% of the initialswrite power levels (P_(ini)), but could also be a pre-defined constant,e.g. 0.1 or 0.2 mW.

In a fifth step 45 the processor 102 performs a pre-defined procedure inorder to derive a characteristic write power level (P_(char)) from thecurve-fitting function of the fourth step. Hence, the characteristicwrite power level (P_(char)) thereby has an associated initial value ofthe write power level (P_(ini)). In the preferred embodiment thecharacteristic value of the write power level (P_(char)) is given by

P _(char)=κ·β,

where κ is a first multiplication constant. κ is read from the opticalrecording medium (1). As the value of P is dependent on the initialvalue of the write power level (P_(ini)) the characteristic value of thewrite power level (P_(char)) is a function of the initial write powerlevel (P_(ini)) in question. In order to assess if the foundcharacteristic value of the write power level (P_(char)) is an optimumvalue a pre-defined procedure is performed where the just obtainedcharacteristic value of the write power level (P_(char)) is evaluatedunder said pre-defined procedure. Hence, the characteristic value of thewrite power level (P_(char)) can be compared to a certain interval ofallowed values and/or compared to certain set of fixed values.

In the most preferred embodiment, the characteristic value of the writepower level (P_(char)) is compared to the corresponding initial value ofthe write power level (P_(ini)) by evaluating the numerical differencebetween the characteristic value of the write power level (P_(char)) andthe corresponding initial value of the write power level (P_(ini)). Ifsaid numerical difference is below a certain threshold level, such as0.1 mW, 0.05 mW, 0.01 mW or 0.005 mW, the characteristic value of thewrite power level (P_(char)) is considered an optimum value and themethod of invention is terminated in the step marked 47 where the writeprocedure is initiated. Said threshold level is advantageously dependenton the power stepping resolution of the writing laser, where the powerstepping resolution may be given by the difference between consecutivelaser power levels during power calibration, i.e. the power differencebetween the points 51 on the horizontal axis in FIG. 3. However, if thecharacteristic value of the write power level (P_(char)) is not anoptimum value according to said pre-defined procedure a sixth step 46 isinitiated.

Numerous different evaluation schemes of the characteristic value of thewrite power level (P_(char)) are readily available to the skilled personin the art, including statistical methods beneficial for reducing thesensitivity of the comparison by inherent scattering of the measuredvalues.

In a sixth step 46 the processor 102 performs a pre-defined procedurefor starting an iteration procedure with the aim of finding an improvedcharacteristic value of the write power level (P_(char)) that meets theconditions of the fifth step 45, i.e. a characteristic value of thewrite power level (P_(char)) that is an optimum value. This is done byan iteration scheme or procedure where the subsequent initial values ofwrite power level (P_(ini, n)), are given by

P _(ini, n+1) =AP _(ini, n)+(1−A)P _(char, n),

where A is a constant, preferably between 0 and 1, and n is an integer.

In FIG. 3, this is schematically illustrated by P_(ini, 2) which isgiven by the above iteration formula with n equal to 1. By the sameprocedure given in the fourth step 44, the measured values 51 of themodulation (M) times power (P) are fitted in a pre-defined intervalaround P_(ini,2) indicated by the arrow 55. Thus, a straight line 56 isobtained, the straight line 56 intersecting with the horizontal axis inthe point 57 having the value β₂. From β₂ a new characteristic value ofthe write power level (P_(char, 2)) is obtained, which is subsequentlyassessed as an optimum value according to fifth step 45 of theinvention. If the obtained characteristic value of the write power level(P_(char, 2)) is not an optimum value, yet another iteration step withmay be taken in order to obtain another characteristic value of thewrite power level (P_(char, 3)) and evaluate if P_(char, 3) qualify asan optimum value. Henceforth, the iteration may proceed until an optimumvalue is obtained.

Thus, for each characteristic value of the write power level(P_(char, i)) obtained, an assessment is made in the fifth step 45. Theloop between the fifth step 45 and the sixth step 46 is preferablycontinued until an optimum value is obtained, but a pre-defined stopprocedure may also break the loop if an optimum value is not obtainedafter a relatively large number of iteration loops.

Numerical calculations have shown that the present invention enables arelatively faster and more stabile iteration process for finding acharacteristic value of the write power level (P_(char)) that isoptimum. Thus, usually just a few iteration steps are needed, andnon-convergence is seldomly observed.

Subsequently, an optimum value (P_(opt)) of the write power level (P),of the radiation beam 5 shown in FIG. 1 can be found. The optimum value(P_(opt)) of the write power level (P) is set to be equal to the optimumvalue of the write parameter times a second multiplication constant (ρ);

P _(opt) =ρ·P _(char).

Hence, an optimum value (P_(opt)) of the write power level (P) forwriting information on the optical recording media is obtained.

It is contemplated that the values of the constants, i.e. A, ρ and κ,may be varied to find an optimum value of the write parameter, possiblyto evaluate the stability of one or more found optimum value(s) of thewrite parameter. Similar, the range fraction (R) may be varied in aniterative manner to assess the found optimum value of the writeparameter and its stability.

Although the present invention has been described in connection with thespecified embodiments, it is not intended to be limited to the specificform set forth herein. Rather, the scope of the present invention islimited only by the accompanying claims. In the claims, the termcomprising does not exclude the presence of other elements or steps.Additionally, although individual features may be included in differentclaims, these may possibly be advantageously combined, and the inclusionin different claims does not imply that a combination of features is notfeasible and/or advantageous. In addition, singular references do notexclude a plurality. Thus, references to “a”, “an”, “first”, “second”etc. do not preclude a plurality. Furthermore, reference signs in theclaims shall not be construed as limiting the scope.

1. A method for setting an optimum value of a write parameter for use inan optical recording apparatus for writing information on an opticalrecording medium (1) by means of a radiation beam (5), the methodcomprising the steps of: 1) writing a series of test patterns on therecording medium, each pattern being written with a different value of awrite power level (P) of the radiation beam, 2) reading the patterns soas to form corresponding read signal portions (18, 19), 3) deriving avalue of a read parameter from each read signal portion, 4)curve-fitting a function defining a relation between the read parameterand the write power level (P), to associated values of the readparameter and the write power level (P), the curve-fitting functionhaving a initial value of write power level (P_(ini,)), 5) obtaining acharacteristic write power level (P_(char)) from the curve-fittingfunction, and assessing if the characteristic write power level(P_(char)) qualify as an optimum value of a write parameter, and if thecharacteristic write power level (P_(char)) does not qualify as anoptimum value of a write parameter then start 6) obtaining one or morecharacteristic write power levels (P_(char, n)) by an iterationprocedure where the subsequent initial values of write power level(P_(ini, n)), are given byP _(ini, n+1) =AP _(ini, n)+(1−A)P _(char, n), where A is a constant,and n is an integer.
 2. A method according to claim 1, wherein the readparameter is a modulation (M) of the amplitude of a read signal derivedfrom information recorded on the recording medium (1).
 3. A methodaccording to claim 2, wherein a curve-fitting functions is of the formP·M=α·(P−β), wherein α and β have values resulting from thecurve-fitting, and wherein the characteristic value of the writeparameter (P_(char)) for the at least one function is set to besubstantially equal to the value of β times a first multiplicationconstant (κ).
 4. A method according to claim 3, wherein the firstmultiplication constant (κ) is read from an area on the recording medium(1) containing control information indicative of a recording processwhereby information can be recorded on said recording medium.
 5. Amethod according to claim 3, wherein the curve-fitting of the functiondefining a straight line is carried out in a predetermined fit range ofwrite power levels.
 6. A method according to claim 5, wherein theposition of the predetermined fit range is determined by the initialvalue of the write power level (P_(ini)) associated with eachcurve-fitting function and a range factor (R).
 7. A method according toclaim 1, wherein the constant A is between 0 and
 1. 8. A method forsetting an optimum value (P_(opt)) of a write power level (P), of aradiation beam, which method is intended for use in an optical recordingapparatus for writing information on an optical recording medium (1) bythe radiation beam (5) having the write power level (P), using a methodas claimed in claim 1 for setting an optimum value of a write parameter,wherein the optimum value (P_(opt)) of the write power level (P) is setto be equal to the optimum value of the write parameter times a secondmultiplication constant (ρ).
 9. A method according to claim 8, whereinthe second multiplication constant (ρ) is read from an area on therecording medium (1) containing control information indicative of arecording process whereby information can be recorded on said recordingmedium.
 10. An optical recording apparatus for implementing the methodaccording to claim 1, the apparatus comprising: a radiation source (4)for emitting a radiation beam (5) having a controllable value of a writepower level (P) for recording information on the recording medium, acontrol unit (12) for recording a series of test patterns, each patternbeing recorded with a different value of the write power level, a readunit (90) for reading the patterns and forming corresponding read signalportions (18,19), first means (10) for deriving a value of a readparameter from each read signal portion, second means (101) forcurve-fitting a function defining a relation between the read parameterand the write power level (P), to associated values of the readparameter and the write power level (P), the curve-fitting functionhaving a initial value of write power level (P_(ini,)), third means(102) for obtaining a characteristic write power level (P_(char)) fromthe curve-fitting function, and assessing if the characteristic writepower level (P_(char)) qualify as an optimum value of a write parameter,and if the characteristic write power level (P_(char)) does not qualifyas an optimum value of a write parameter then initiate, fourth means(102) for obtaining one or more characteristic write power levels(P_(char, n)) by an iteration procedure where the subsequent initialvalues of write power level (P_(ini, n)), are given byP _(ini, n+1) =AP _(ini, n)+(1−A)P _(char, n), where A is a constant,and n is an integer.
 11. An optical recording medium (1) for recordinginformation by irradiating the recording medium by means of a radiationbeam (5), the recording medium comprising an area containing controlinformation indicative of a recording process whereby information can berecorded on said recording medium, the control information comprisingvalues of recording parameters for the recording process, wherein thecontrol information comprises a information indicative of the initialwrite power level (P_(ini)) for use in the method as claimed in claim 1.12. An optical recording medium (1) for recording information byirradiating the recording medium by means of a radiation beam (5), therecording medium comprising an area containing control informationindicative of a recording process whereby information can be recorded onsaid recording medium, the control information comprising values ofrecording parameters for the recording process, wherein the controlinformation comprises at least one of the constant A, the firstmultiplication constant (κ) or the second multiplication constant (ρ)for use in the method as claimed in claim 1.